1. Technical Field of Invention
The present invention relates to a metal halide lamp suitable for typical use as a light source in vehicular headlights.
2. Prior Art
Because of their ability to provide illumination at high luminance, metal halide lamps are increasingly used today as a light source in vehicular headlights and other applications.
The construction of a conventional metal halide lamp for typical use in vehicular headlights is shown in FIG. 5. It comprises a discharge vessel 104 forming a generally ellipsoidal discharge space 102 extending in a longitudinal direction and a pair of electrodes 106A, 106B embedded in the discharge vessel 104 at the narrowest end portions of the discharge space 102. The tips of electrodes 106A, 106B project into the discharge space 102. Mercury, a starter gas and a metal halide are enclosed in the discharge space 102.
The metal halide is enclosed in order to enhance the lamp efficiency and color rendering. The amount of the metal halide being enclosed is set to provide a predetermined luminous flux and light color, while ensuring that no excess amount will affect the luminous intensity distribution pattern. Specifically, if the discharge space 102 has a capacity of about 30 xcexcl, the amount of the metal halide enclosed should range from about 0.45 to 0.6 mg (from about 0.015 to about 0.02 mg/xcexcl when calculated for the amount enclosed per unit volume).
A problem with the conventional metal halide lamp described above is that an arc often fizzles out after the lamp was switched on.
When the lamp is not on, a metal halide 108, as shown in FIG. 5, is deposited in the lowest area of the inner surface of the discharge vessel 104, which is midway between the narrow end portions of the discharge vessel 104. The metal halide 108 evaporates when the lamp is turned on. If the amount of metal halide 108 enclosed in the discharge space 102 is excessive compared to the volume of the discharge space 102, the metal halide deposit 108 in the lowest area of the inner surface of the discharge vessel 104 is very close to the electrodes 106A, 106B. Even if a high voltage is applied between these electrodes, a portion of the arc developed between the electrodes 106A, 106B is prone to move toward the metal halide 108, rather than growing until it bridges the two electrodes 106A, 106B. If this xe2x80x9cshuntingxe2x80x9d occurs, the effective impedance between the electrodes 106A, 106B decreases to such an extent that the arc does not grow but fizzles out.
Thus, the conventional metal halide lamp often fails to come on instantaneously with application of a high voltage between the electrodes 106A, 106B and conventional metal halide lamp must be ignited several times to come on. This is not very desirable if the metal halide lamp is to be used in vehicular headlights and other applications that must produce illumination instantaneously.
The present invention has been accomplished under these circumstances and has as an object providing a metal halide lamp that is protected against fizzling out of an arc after it was switched on.
According to the present invention, the stated object is attained by appropriately adjusting the amount of the metal halide enclosed in the discharge space.
The present invention provides a metal halide lamp comprising a discharge vessel that forms a generally ellipsoidal discharge space extending longitudinally and a pair of electrodes embedded in said discharge vessel at the narrowest portions of said discharge space in such a way that their tips project into said discharge space, with mercury, a starter gas and a metal halide enclosed in said discharge space, characterized in that the amount of said metal halide being enclosed per unit volume of said discharge vessel is set within a range of 0.006-0.01 mg/xcexcl.
The specific composition of the xe2x80x9cstarter gasxe2x80x9d to be used in the invention is not limited. Xenon and argon gases are suitable starter gases.
The specific composition of the metal halide to be used in the invention also is not limited. Halides of metals such as thallium, sodium, indium and scandium, as well as mixtures thereof are suitable for use as the metal halides.
As noted above, the metal halide lamp of the present invention which has the metal halide enclosed in the generally ellipsoidal discharge space together with mercury and the starter gas is characterized in that the amount of the metal halide enclosed in unit volume of the discharge space is advantageously set in the range 0.006-0.01 mg/xcexcl.
If the amount of metal halide enclosed in the discharge space does not exceed 0.01 mg/xcexcl, its deposit on the inner surface of the discharge vessel in the lowest area midway between the right and left sides thereof will not come close enough to either electrode to cause xe2x80x9cshuntingxe2x80x9d. This prevents the fizzling out of an arc after the lamp was switched on. However, if the amount of metal halide enclosed in the discharge space is less than 0.006 mg/xcexcl, the metal halide lamp can no longer produce the intended luminous flux and color of light.
Thus, the metal halide lamp of the invention which has the metal halide enclosed in the discharge space in an amount of 0.006-0.01 mg/xcexcl produces the intended luminous flux and color of light, and yet successfully prevents the fizzling out of an arc after the lamp was switched on.
A word must be said about the above-defined range of the amount of the metal halide to be enclosed in the discharge space per unit volume. In certain circumstances, such as where the discharge space is an extremely oblong ellipsoid, the deposit of the metal halide on the inner surface of the discharge vessel in its lowest area midway between the right and left sides thereof can come unduly close to either electrode, even if the above-defined range is observed.
In the invention, the distance L from the tip position of each electrode to the position on the inner surface of the discharge vessel at its lowest area midway between the narrow end portions thereof, along with the input power P to the metal halide lamp, is adjusted such that the ratio L/P is set in the range 0.05-0.1 mm/W. This ensures that the deposit of the metal halide on the inner surface of the discharge vessel in its lowest area midway between the narrow end portions thereof will not come unduly close to either electrode.
If the ratio L/P is greater than 0.1 mm/W, the distance between each electrode and the position on the inner surface of the discharge vessel at its lowest portion midway between the narrow end portions thereof is very great. Thus, even when the lamp is on, the temperature in that position does not rise sufficiently to create adequate light emission. The lamp, in turn, fails to produce the desired luminous flux and light of color. Since the input power P is generally proportional to the capacity of the discharge space, the ratio L/P has the advantage of using a straightforward index keyed to the size of the discharge vessel to determine input power.